Modelling hydrological processes and aluminium leaching in an acid soil at Llyn Brianne, mid-Wales

Recent research has shown that enhanced acid deposition on to the canopies of commercial conifer forests in upland Wales has contributed to the release of Al into soil and stream waters. Relatively little is known about how soilhydrological and chemical processes interact in response to acid deposition and afforestation to regulate the transfer of Al-rich soil water into streams. A field-based hydrochemical study monitored the hydrology and soilwater chemistry of an afforested stagnopodzol at LlynBrianne (Mid-Wales) during a hydrological year. Water moved vertically through the soil profile, aided by root-promoted macropores. Soil water fluxes, estimated by the LEACHM model, indicated that 89% of effective precipitation drained vertically through the soil profile. Precipitation reaching the soil was acidified and enriched in anions during its passage through the forest canopy. Further acidification occurred as water moved into the soil and Al was mobilized, probably by cation exchange processes. Leaching losses of Al from the soil profile occurred during all major hydrological events and the total flux was estimated at 3.39 kmolc ha−1 year−1. It is suggested that Al-rich soil water draining from the base of the stagnopodzol profile can provide a significant input to streams during acid episodes.